1. Field of the Invention
The present invention relates to aluminum door beams used in reinforcing members for doors of vehicles, such as automobiles and trucks. The door beam is arranged in a door to absorb the shock from a collision in the side direction and to secure safety of passengers.
2. Description of the Related Art
Recently, the global environment has been regarded as being of worldwide importance. For example, regulations for reducing gas emissions including carbon dioxide from automobiles have been strengthened in many countries in order to suppress global warming. Accordingly, lightweight automobiles have been in rapid development.
A door beam for an automobile is attached to the interior of a door in order to absorb the shock from a collision. A typical conventional material used is steel, for example, high-tensile steel of 150 kgf/mm2 grade. In recent years, however, the use of aluminum extrusions has been investigated in view of achievement of a lightweight automobile.
Door beams for automobiles (also referred to as impact beams, impact bars, guard bars, or door side beams) are required to have high energy absorbability to soften the shock from a collision. For example, Federal Motor Vehicle Safety Standard (FMVSS) defines criteria of the bending strength and absorbed energy to a load applied from the side of a vehicle. At laboratory tests, these bending properties are evaluated by a three-point bending strength test simulating side collision of a vehicle as shown in FIG. 2A, in which a door beam is supported at the two ends and a load is applied to the center.
FIG. 2B is a typical schematic load (P) vs. displacement (xcex4) curve in the three-point bending test shown in FIG. 2A. FIG. 2B shows that the load reaches a maximum value as the displacement increases, and then it decreases at a further displacement because of overload buckling of the aluminum beam. In general, it is preferred that the maximum load be larger and the displacement when the buckling occurs be larger, that is, the energy absorption be larger, as shown by a solid line in FIG. 3. The energy absorption corresponds to the area represented by hatched lines in the load (P) vs. displacement (xcex4) curve of FIG. 2B.
Stricter properties have been required for door beams being highly conscious of safety, that is, improvements in maximum load and energy absorption without an increase in the weight have been required. For example, in a three-point bending test under a specified condition for door beams, a current required level of the maximum load is 1,300 kg, which is considerably higher than the conventional level 1,100 kg.
Recently, door beams have been applied to compact cars having short doors. Since the distance (L) between the two ends in FIG. 2A is short, in collision of compact cars, a small displacement (xcex4) causes a larger bending curvature. Thus, rupture will occur more readily with a small displacement.
The present inventors have actively investigated a technology for achieving an aluminum door beam without an increase in weight, which has a large maximum load, a large displacement before buckling (hereinafter referred to as buckling displacement), a large displacement without rupture, and a large energy absorption in view of a cross-section and dependence of the surface texture on the composition of the door beam material.
The investigation was performed in view of the following two aspects. First, the rupture of the door beam causes decreased absorption energy, and the ruptured portion is harmful for the passenger. Thus, the rupture must be absolutely avoided. A target of the present invention is to provide a configuration in which buckling proceeds predominantly before the inner flange at the extension side breaks by the limit of stress-strain characteristics.
Second, another possible method to prevent the rupture of the door beam is increased thicknesses of the flange and the web; however, this method caused an increase in weight. Thus, another target of the present invention is to control the composition and the surface texture of the door beam material for simultaneously achieving lightweight and high performance.
As a result, the present inventors have made the following finding. In the cross-section of an aluminum door beam, the radius R of the outer corner at the extended ends of the outer flange (hereinafter referred to as RFO) and the radius R of the outward corner at the connections between the web and the outer and inner flanges (hereinafter referred to as RWO) significantly affect the buckling displacement and energy absorption in the load (P) vs. displacement (xcex4) curve. In the dependence of the surface texture on the composition of the door beam material, when the thickness of the recrystallization layer on the outer surface of the door beam is reduced or the layer is eliminated, the stress concentration during bending deformation is prevented and the energy absorption is improved. This is prominent in a door beam having a large maximum load.
The present invention is achieved based on the finding.
Accordingly, it is an object of the present invention to provide an aluminum extruded door beam comprising an outer flange, an inner flange, and at least one web for connecting the outer flange and the inner flange, the outer corners at the extended ends of the outer flange having a radius R of 2.5 mm or less.
It is another object of the present invention to provide an aluminum extruded door beam material comprising an outer flange, an inner flange, and at least one web for connecting the outer flange and the inner flange, the outward corners at the connections between the web and the inner flange and between the web and the outer flange having a radius R of 2 mm to 4 mm.
It is a further object of the present invention to provide an aluminum extruded door beam material comprising an outer flange, an inner flange, and at least one web for connecting the outer flange and the inner flange, the radius of the outward corners at the connections between the web and the inner flange and between the web and the outer flange being 1.5 to 2 times the width of the web.
It is a still further object of the present invention to provide an aluminum extruded door beam material comprising an outer flange, an inner flange, and at least one web for connecting the outer flange and the inner flange, the length of the extended ends of the outer flange being 1 to 2 times the radius R of the outward corner at the connections between the web and the flanges.
It is still another object of the present invention to provide an aluminum alloy extruded door beam material comprising 0.8 to 1.5% by weight (hereinafter the same) of Mg; 4 to 7% of Zn; 0.005 to 0.3% of Ti; at least one element selected from the group consisting of 0.05 to 0.6% of Cu, 0.2 to 0.7% of Mn, 0.05 to 0.3% of Cr, and 0.05 to 0.25% of Zr; and the balance being Al and incidental impurities, the thickness of the recrystallization surface layer being 50 xcexcm or less.
It is a still further object of the present invention to provide an aluminum alloy extruded door beam material comprising 0.8 to 1.5% by weight (hereinafter the same) of Mg and 4 to 7% of Zn, the recrystallization surface layer having a thickness of 50 xcexcm or less.